Meltable materials, commonly called hot melts, find increasing use in industry for instance as temporary or permanent fusion adhesives or for coating substrates. Hot melts may be used to coat substrates to provide, for instance, a gas impermeable surface thereof or, for instance, for the production of adhesive tapes, adhesive foils, for self-sealing labels. Hot melts are also commonly used for such purposes as cementing shoe and leather parts to each other, to glue cardboards to each other, to glue the back of the books to the remainder thereof, to glue parts of furniture to each other, as well as during other assembly and finishing operations.
The known hot melts are predominantly binary and tertiary mixtures of basic polymers, adhesive resins and waxes, softeners and fillers. Hot melts and adhesives are available in various piecemeal forms such as granulate, powder, slivers, pearls, strands, candles, and in bulk solid form such as blocks and slugs. These materials are prepared for use by liquifying them by heating in a liquifying apparatus and by pumping or otherwise conveying the liquified hot melt to an application head in application apparatus which applies the hot melt to the surfaces to be coated or adhesively joined.
There are two principal types of dispensing apparatus which are currently in use to liquify and dispense hot melt. A first type of apparatus, which can be termed a "hopper" dispenser, includes a hopper, a melter for liquifying hot melt deposited in the hopper, and a pump to pump the liquified material from the melter to the application head. This type of dispenser is typically used in low volume hot melt operations in which hot melt is fed to the melter in piecemeal type forms in quantities weighing twenty lbs. or less. These small quantities can be fed into the hopper simply by "pouring" the hot melt pieces from the container into the hopper.
The other type of dispensing apparatus is known as a drum unloader. A drum unloader liquifies and dispenses solid, unitary mass, slug form hot melt directly from a 55-gallon or (equivalent sized metric drum container) or from pail sized containers. A drum unloader generally comprises a hydraulically driven platen including a melting grid, a dispensing aperture in the platen and a pump to pump the hot melt from the aperture to the application. In operation, the platen is pressed into the open end of an upright drum or pail of hot melt and the hot melt is liquified by the melting grid and forced up and out of the dispensing aperture by the pressurizing force of the platen into the pump. Drum unloaders are typically used in larger volume operations due to the volume of hot melt needed in the operation and because hot melt is typically less expensive when purchased in solid slug form in drum quantities. In addition, certain hot melts are so viscious they cannot be pumped to the application without the aid of hydraulic pressure, and thus drum unloaders are uniquely qualified for dispensing hot melts of this type.
Drum unloaders do, however, suffer from several drawbacks. The most serious of these drawbacks is the rupturing of the drum containers during use, whereby extremely hot and dangerous hot melt is spewed from the ruptured drum. Such ruptures can occur either as the result of defects in the drum walls or as the result of equipment malfunction and/or operator error. In addition, drum unloaders tend to leave a measurable quantity of hot melt in the bottom of the drum which cannot be forced out by the platen due to unavoidable voids between the platen and the bottom of the drum. Extracting this residue from the drum is difficult and time consuming, and as such the residue is often discarded with the drum, resulting in undesirable losses. Drum unloaders also suffer from a problem called "burying the platen". This problem occurs when the seal between the platen and the side surface of the drum is broken so that hot melt flows up and over the platen, thus "burying" it in liquified hot melt. Extracting a buried platen from a hot melt drum is difficult, time consuming and often dangerous. Another drawback of a similar nature results from the need to remove the hot platen from the drum for changover to a new drum or at the end of the workshift. Hot, liquified hot melt often drips from the platen when it is lifted out of the drum exposing workers to the risk of a serious burn and creating a difficult mess in any event. Drum unloaders also require that the application process be stopped to switch drums resulting in undesirable process down time. Moreover, certain types of hot melt are classified as hazardous materials. If an emptied drum contains a significant quantity of such hazardous materials as they usually do, its disposal requires special, more expensive procedures than required for disposal of unhazardous waste.
In view of the many drawbacks to drum unloaders others have recognized it would be beneficial to provide a hopper-type melter system that could accommodate drum or pail quantities of slug form hot melt. The economies of purchasing hot melt in drum quantities and slug form are thus achieved while the disadvantages of drum unloaders are avoided. However, unloading a slug of hot melt from a drum or pail is very difficult because the viscosity and stickiness of unheated hot melt makes it impossible to pour or slide out of the container.
At least two systems have been proposed to deal with the problem of unloading a container of slug form hot melt into a hopper. In U.S. Pat. No. 4,505,669 to Rogers there is disclosed a hopper-type melter which contemplates loading the entire hot melt drum, including the container, into the hopper. To melt the hot melt out of the drum container, the Roger's device includes heating elements which project upwardly from the bottom of the hopper so that they extend into the lower end of the drum container positioned in the dispenser and make contact with the hot melt material held therein. These projections melt the hot melt sufficiently to cause it to flow downwardly to the bottom of the melter where additional heating elements are provided to liquify it sufficiently to be pumped to the application heads. While Rogers also indicates that hot melt may be placed in the hopper without its drum container as a solid unitary mass, it does not suggest a way to get the hot melt out of the container and into the hopper.
The system of Rogers has several drawbacks. The main problem results from leaving the drum container in the unit while the hot melt is dispensed. If the hot melt melts out of the drum faster than it is pumped out and dispensed it backs up in the space between the outside of the drum and the hopper. Removing the drum from the hopper when it is empty thus becomes extremely difficult, particularly if hot melt cools and congeals in the space between the hopper and drum. Moreover, when the drum is removed its outside is coated with hot melt, making it difficult to handle. Furthermore, it is possible that the backed-up liquified hot melt could get contaminated with paint or other foreign substances from the outside surface of the drum container, possibly altering its adhesive characteristics. Moreover, unless the heating elements of the Roger's device can supply sufficient heat to the "top" end of a drum being unloaded it is possible that a quantity of hot melt will remain stuck in the top end of the drum and fail to be melted out and released to the "bottom" of the drum for liquification by the heating elements. As said above, however, applying too much heat with the heating elements can cause unwanted back-up of the hot melt.
Another approach to dispensing drum quantities of hot melt from hopper-type dispensers has been provided by the Meltex Corporation of Peachtree City, Ga. The Meltex approach utilizes a bulk hopper-type melter having a capacity sufficient to accommodate a 55-gallon (or equivalent metric size) drum of hot melt and a drum unloading device which tips up and lifts a drum over the melter for unloading. Once tipped up over the hopper the outside of the drum is radiated with infrared heaters mounted over the melter until the hot melt slug inside the drum releases from the container into the melter's hopper. To control the direction and rate of the slide of the slug into the hopper the Meltex unit includes a heated spike positioned across the diameter of the open end of the drum. The spike restrains the movement of the hot melt slug to the rate of melting of the spike through the slug. Thus, the Meltex system is somewhat elaborate in design and unwieldy to use. In addition, because the system involves suspending a large mass of semi-liquified hot melt from a considerable height it is inherently dangerous.
The present invention, like the Rogers and Meltex devices, provides a hopper-type dispenser for use in dispensing drum quantities of slug form hot melt. The invention is similar to the Meltex system in that it provides means for uloading the contents of a hot melt drum into the hopper of the dispenser, as opposed to the Rogers' system which contemplates leaving the container in the melter while dispensing its contents. The present invention is, however, simpler in construction than the Meltex system, can empty the contents of hot melt drum into the hopper in less time than the Meltex system, and assures that the entire hot melt slug is safely deposited in its entirety in the hopper without risk of spilling or splashing.